Fungi of the genus Trichoderma are present in soils as well as in other diverse habitats. They are beneficial symbiotic partners for plants, particularly crops. Trichoderma spp. secrete cellulases and hemicellulases to degrade β-glucan and xylan, the key structural components of lignocellulosic biomass, into glucose and xylose, respectively. Several high-enzyme-producing strains (e.g., QM9414, Rut-C30) in use today have been generated from Trichoderma reesei QM6a strain by treatments with chemical mutagens and/or radiation and widely used for industrial applications. However, due to multiple rounds of chemical and/or physical mutagenesis, the genomes of these hypersecretion mutants have plenty of mutations, deletions and rearrangements [1-3], which cause genome instability. Further, these industrial strains secret less xylan-degrading hemicellulases than β-glucan-degrading cellulases.
T. reesei is the anamorph of the pantropical ascomycete Hypocrea jecorina [4]. H. jecorina CBS999.97 wild isolate undergoes a heterothallic reproductive cycle, and generates CBS999.97(1-1) and CBS999.97(1-2) haploids with MAT1-1 and MAT1-2 mating-type loci, respectively. QM6a has a MAT1-2 mating type locus and can also mate with the CBS999.97(1-1) haploid to form fruiting bodies that contain asci with 16-part ascospores [5].
Meiosis is a special type of cell division that gives rise to genetic diversity in sexually reproductive organisms. Programmed DNA double-strand breaks (DSBs) are spontaneously generated throughout the genome by the meiosis-specific Spo11 endonucleases [6]. In model organisms like yeast and mouse, the Spo11-induced DSBs are repaired robustly by error-free homologous recombination to ensure accurate segregation of homologous chromosomes and genome stability. By contrast, previous studies have revealed that aneuploid or segmentally aneuploid (SAN) meiotic products are generated in some filamentous fungi. For example, in the fungal human pathogen Cryptococcus neoformans, a large segmental duplication occurs during meiosis via telomere-telomere fusion and chromosomal translocation between two different chromosomes [7,8]. The plant-pathogenic fungus Mycosphaerella graminicola (anamorph Septoria tritici) generates ascospores with up to eight dispensable chromosomes [9,10]. Some ascospore isolates of the plant-pathogenic fungus Nectria haematococca mating population VI (anamorph Fusarium solani) contain “extra” chromosomes, called “conditionally dispensable” chromosomes [11-13]. Nectria haematococca mating population VI is notable also for being a genetically close species of T. reesei in the order Hypocreales [13,14].
There is still a need to develop a technology to obtain new strains of T. reesei that exhibit enhanced expression or activities of carbohydrate degrading enzymes, especially hemicellulases.